Liquid crystal display device

ABSTRACT

The present invention provides, by using a photo-alignment film, a liquid crystal display device in which a good voltage holding ratio is retained for a long period of time and occurrence of image sticking and stain in a display screen is prevented. The liquid crystal display device of the present invention includes an active matrix liquid crystal panel and a backlight, the liquid crystal panel including a liquid crystal layer, a pair of substrates that sandwich the liquid crystal layer in between, and an alignment film disposed on a liquid crystal layer side surface of each of the substrates, each alignment film being a photo-alignment film formed from a material that exhibits a photo-alignment characteristic and containing carboxyl groups on the liquid crystal layer side, the liquid crystal layer containing a liquid crystal material and a nitrobenzene derivative.

TECHNICAL FIELD

The present invention relates to a liquid crystal display device. Morespecifically, the present invention relates to a liquid crystal displaydevice in which alignment of liquid crystal molecules is controlled byan alignment film.

BACKGROUND ART

A liquid crystal display device is a display device in which a liquidcrystal composition is used for display, and a representative displaymethod thereof is controlling the amount of light transmitted through aliquid crystal panel in which the liquid crystal composition is sealedbetween a pair of substrates by irradiating the liquid crystal panelwith light from a backlight and by applying voltage to the liquidcrystal composition to change the alignment of liquid crystal molecules.Such a liquid crystal display device has merits such as being thin andlight and consuming a small amount of electricity, and thus is used forelectronic devices such as smartphones, tablet PCs, and car navigationsystems. In recent years, the definition of pixels has increased for useof, for example, a smartphone, and there has been a tendency that thenumber of conductive lines and the area of the black matrix provided inthe liquid crystal panel increase along with this.

Typically, in a liquid crystal display device, the alignment of liquidcrystal molecules in a state in which no voltage is applied iscontrolled by an alignment film that has been subjected to alignmenttreatment. Conventionally, as a method of alignment treatment, a rubbingmethod of rubbing the surface of the alignment film by, for example, aroller, has been widely used. However, since the number of conductivelines and the area of the black matrix provided in a liquid crystalpanel have increased, steps have become more likely to be formed in thesurface of a substrate in the liquid crystal panel. If steps are presentin the surface of the substrate, the vicinity of the steps cannot berubbed appropriately by using a rubbing method in some case. In the casewhere alignment treatment is not uniformly performed, decrease of acontrast ratio of the liquid crystal display device is caused.

To address this, in recent years, research and development have beenpromoted on a photo-alignment method of irradiating the surface of analignment film with light as a method of alignment treatment thatreplaces the rubbing method. According to the photo-alignment method,alignment treatment can be performed without contact with the surface ofthe alignment film. Therefore, the alignment treatment is less likely tobe uneven even in the case where steps are present in the surface of thesubstrate, and a merit that good liquid crystal alignment can berealized on the entire surface of the substrate can be achieved.

In addition, the increase of the number of conductive lines and the areaof the black matrix provided in the liquid crystal panel decreases thearea ratio (opening ratio) of an opening portion that can be used fordisplaying. The decrease of the aperture ratio directly leads todecrease of the amount of light that can be transmitted through theliquid crystal panel, and thus greatly improving the luminance of abacklight in order to retain the display performance of the liquidcrystal display device in terms of, for example, a contrast ratio, hasbeen considered.

Meanwhile, with regard to a liquid crystal composition used for a liquidcrystal display device, it has been desired that the stability thereofis enhanced such that the liquid crystal composition endures a loadduring a production process of the liquid crystal display device and aproduced liquid crystal display device can exhibit a stablecharacteristic for a long period of time. For example, Patent Literature1 discloses adding a hindered amine photostabilizer to a liquid crystalcomposition to suppress decomposition of the liquid crystal compositioncaused by heating and exposure to ultraviolet light and improve thereliability of the liquid crystal display device particularly in termsof temporal change of a voltage holding ratio. In addition. PatentLiterature 2 discloses that a liquid crystal composition that has a highvoltage holding ratio and is stable against heat and light can beobtained by adding one, two, or more kinds of phenolic antioxidants to aliquid crystal compound having a negative anisotropy of dielectricconstant.

CITATION LIST Patent Literature

-   Patent Literature 1: JP 2012-224632A-   Patent Literature 2: WO 2013/187373

SUMMARY OF INVENTION Technical Problem

As described above, to address the increase in the definition of pixels,using the photo-alignment method and increasing the luminance of thebacklight have been attempted. However, it has been revealed that stain(unevenness) is likely to occur at an end portion of the screen of theliquid crystal panel and an end portion of a displayed box pattern as aresult of the attempts. It is to be noted that a malfunction of the endportion of the displayed box pattern is detected as image sticking. Inaddition, it has been also revealed that there is a tendency that thewidth of a seal material attaching the substrates together is set to besmaller to increase the ratio of a screen in the liquid crystal panel,and that stain is likely to occur in the periphery of the liquid crystalpanel.

As a result of various investigations, the present inventors consideredthat the image sticking and stain described above occur through thefollowing steps.

(1) Generation of Radical

As a result of the liquid crystal panel being irradiated with light fromthe backlight (amount of energy: hv), as shown in a formula (A-I),photofunctional groups contained in a photo-alignment film are excitedand cleave, and thus radicals are generated. Particularly, in the casewhere a backlight whose luminance is increased is used, the generationof radicals is more prominent.

(2-1) First Generation of Ions

Radicals generated in the photo-alignment film are eluted into a liquidcrystal layer, and the eluted radicals are ionized.

(2-2) Second Generation of Ions

Radicals generated in the photo-alignment film are eluted into theliquid crystal layer and are transferred from the photofunctional groupsto liquid crystal molecules, and thus the liquid crystal molecules areionized.

(2-3) Third Generation of Ions

Neutral impurities are ionized by moisture coming from the outside ofthe liquid crystal panel.

(3) Decrease of Voltage Holding Ratio

Ions in the liquid crystal layer stagnate in the end portion of thescreen of the liquid crystal panel and the end portion of the displayedbox pattern, and the voltage holding ratio (VHR) of the portionsdecreases, and thus the image sticking and stain described above occur.

It is to be noted that, as described above, there is a conventionalliquid crystal composition to which additives such as antioxidants andphotostabilizers are added. However, these additives do not solve amalfunction unique to a case of using a photo-alignment film. That is,in a liquid crystal display device, when oxygen enters the liquidcrystal panel from outside and a liquid crystal material is oxidized,sometimes image sticking and stain of displayed screen are caused byoxides. To prevent this, conventionally, an antioxidant or the likeadditive that has a function of eliminating oxygen from an oxidegenerated by light or heat in the presence of oxygen has been added to aliquid crystal composition. However, in the case where radicals aregenerated from a photo-alignment film and the radicals are reacted withthe antioxidant, the antioxidant is consumed, and thus the antioxidantcannot play the role that the antioxidant is supposed to play andoxidation of the liquid crystal molecules and the alignment filmprogresses. Oxides generated by this are also sometimes ionized, whichalso causes the decrease of VHR. Further, an alignment film formed froma material such as polyamic acid contains carboxyl groups, and somephotostabilizer reacts with carboxyl groups exposed on the liquidcrystal layer side to generate ionic impurities. Such ionic impuritiesalso cause the decrease of VHR.

In view of the above state of the art, it is an object of the presentinvention to provide, by using a photo-alignment film, a liquid crystaldisplay device in which a good voltage holding ratio is retained for along period of time and occurrence of image sticking and stain in adisplay screen is prevented.

Solution to Problem

The present inventors focused on the fact that, in a liquid crystaldisplay device including a photo-alignment film, the decrease of voltageholding ratio occurs at an end portion of a screen of a liquid crystalpanel and an end portion of a displayed box pattern and this causesmalfunctions such as image sticking and stain in a display screen.Therefore, as a result of an extensive study, the present inventorsbecame the first to find out that the cause of the malfunctions iselution of radicals generated in the photo-alignment film exposed tolight from the backlight into the liquid crystal layer. In addition, thepresent inventors carried out further studies, and found out that anitrobenzene derivative has a high reactivity with radicals, does notproduce ionic impurities with a member such as an alignment film, andcan trap moisture coming from the outside. As a result of this, thepresent inventors found that the problem described above can bebeautifully solved by containing, in a liquid crystal layer, anitrobenzene derivative as a radical scavenger, and successfully reachedthe present invention.

That is, an embodiment of the present invention may be a liquid crystaldisplay device of the present invention that includes an active matrixliquid crystal panel and a backlight, the liquid crystal panel includinga liquid crystal layer, a pair of substrates that sandwich the liquidcrystal layer in between, and an alignment film disposed on a liquidcrystal layer side surface of each of the substrates, each alignmentfilm being a photo-alignment film formed from a material that exhibits aphoto-alignment characteristic and containing carboxyl groups on theliquid crystal layer side, the liquid crystal layer containing a liquidcrystal material and a nitrobenzene derivative.

Advantageous Effects of Invention

According to the liquid crystal display device of the present invention,since the liquid crystal display device has the configuration describedabove, radicals eluted into the liquid crystal layer can be deactivatedby a nitrobenzene derivative and further moisture coming from theoutside can be trapped, and thus decrease of voltage holding ratio canbe prevented. As a result of this, a good voltage holding ratio can beretained for a long period of time by using a photo-alignment film, andoccurrence of image sticking and stain in a display screen can beprevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a liquid crystal displaydevice of the present embodiment.

FIG. 2 is a diagram illustrating a reaction mechanism of a nitrobenzenederivative deactivating a radical generated in a photo-alignment film.

FIG. 3 is a diagram illustrating a reaction of a nitrobenzene derivativewith a water molecule.

FIG. 4 is a diagram illustrating an effect of a phenolic antioxidant inthe present invention.

FIG. 5 is a diagram illustrating a reaction of a phenolic antioxidantwith a photo-alignment film.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described below. Thepresent invention is not limited to what will be described in theembodiment below, and the design thereof can be appropriately modifiedwithin a range that satisfies the elements of the present invention.

FIG. 1 is a schematic cross-sectional view of a liquid crystal displaydevice of the present embodiment. A liquid crystal display device of thepresent embodiment includes an active matrix liquid crystal panel 20 anda backlight 10. The liquid crystal panel 20 includes a liquid crystallayer 23, a pair of substrates 21 that sandwich the liquid crystal layer23 in between, and an alignment film 22 disposed on a liquid crystallayer 23 side surface of each of the substrates 21. Each alignment film22 is a photo-alignment film formed from a material having aphoto-alignment characteristic, and the liquid crystal layer 23 containsa liquid crystal material and a nitrobenzene derivative.

Any active matrix liquid crystal panel may be used as the active matrixliquid crystal panel 20 as long as the active matrix liquid crystalpanel 20 includes the liquid crystal layer 23, the pair of substrates 21that sandwich the liquid crystal layer 23 in between, and the alignmentfilm 22 disposed on the liquid crystal layer 23 side surface of each ofthe substrates 21, and a normal liquid crystal panel employing an activematrix display system can be used. In an active matrix display system,normally, when an active element such as a thin-film transistor (TFT)provided in each pixel is on, a signal voltage is applied to thecorresponding electrodes through the TFT, and an electrical chargecharged in the pixel at this time is held during a period in which theactive element is off. A voltage holding ratio (VHR) indicates a ratioof a charged electrical charge held in a period of one frame (forexample, 16.7 ms). That is, a low VHR means that a voltage applied tothe liquid crystal layer is likely to attenuate as time passes, and itis desired to increase the VKR in an active matrix display system.

Examples of the pair of substrates 21 include a combination of an activematrix substrate (TFT substrate) and a color filter substrate (CFsubstrate). As the active matrix substrate, an active matrix substratethat is normally used in the field of liquid crystal display device maybe used. Examples of a configuration of the active matrix substrate in aplan view include a configuration that includes, on a transparentsubstrate, plural parallel gate signal lines; plural source signal linesextending in a direction perpendicular to the gate signal lines andformed to be parallel to one another; active elements such as TFTsdisposed at positions corresponding to points of intersection of thegate signal lines with the source signal lines; pixel electrodesarranged in matrix in regions defined by the gate signal lines and thesource signal lines; and so forth, in the case of a horizontal alignmentmode, common lines; counter electrodes connected to the common lines;and so forth are further provided on the TFT substrate. As the TFTdescribed above, a TFT in which a channel is formed by using an oxidesemiconductor is preferably used. As the oxide semiconductor, forexample, a compound (In—Ga—Zn—O) constituted by indium (In), gallium(Ga), zinc (Zn), and oxygen (O), a compound (In-Tin-Zn—O) constituted byindium (In), tin (Tin), zinc (Zn), and oxygen (O), or a compound(In—Al—Zn—O) constituted by indium (In), aluminum (Al), zinc (Zn), andoxygen (O) may be used.

As the color filter substrate described above, a color filter substratethat is normally used in the field of liquid crystal display device maybe used. Examples of the configuration of the color filter substrateinclude a configuration in which a black matrix formed in a latticeshape, a color filter formed Inside the squares in the lattice, that is,pixels, and so forth are provided on a transparent substrate. In thecase of a vertical alignment mode, common lines; common electrodesconnected to the common lines; and so forth are further provided on thecolor filter substrate.

It is to be noted that, the pair of substrates 21 may be a pair ofsubstrates on one of which both color filters and an active matrix areformed.

In addition, the alignment films 22 are interposed between the pair ofsubstrates 21 and the liquid crystal layer 23. The alignment films 22have a function of controlling the alignment of liquid crystal moleculesin the liquid crystal layer 23, and, when a voltage applied to theliquid crystal layer 23 is less than a threshold voltage (including whenno voltage is applied), the alignment of the liquid crystal molecules inthe liquid crystal layer 23 is controlled mainly by the alignment films22. An angle formed by a longitudinal axis of each liquid crystalmolecule and the surfaces of the substrates 21 in this state is referredto as a “pre-tilt angle”. It is to be noted that, in this description, a“pre-tilt angle” indicates an angle of inclination of the liquid crystalmolecules from a direction parallel to the surfaces of the substrate, anangle parallel to the surface of the substrates is 0°, and the angle ofa normal of the surface of each substrate is 90°.

The value of the pre-tilt angle of the liquid crystal molecules impartedby the alignment films 22 is not particularly limited, and the alignmentfilms 22 may be horizontal alignment films and may be vertical alignmentfilms. In the case where a display mode is a horizontal alignment modesuch as a fringe field switching (FFS) mode or an in-plane switching(IPS) mode, horizontal alignment films are preferably used. In the casewhere horizontal alignment films are used, it is preferable that thepre-tilt angle is substantially 0° (for example, smaller than 10°), andit is more preferable that the pre-tilt angle is 0° from the viewpointof achieving an effect of retaining a good contrast characteristic for along period of time. Particularly, in the case of the FFS mode or theIPS mode, it is preferable that the pre-tilt angle is 0° also from theviewpoint of a viewing angle characteristic. However, in the case wherethe display mode is a twisted nematic (TN) mode, the pre-tilt angle isset to, for example, about 2° due to the restriction of the mode. Incontrast, in the case where the display mode is a vertical alignmentmode such as a 4-domain reverse twisted nematic (4D-RTN) mode, a4-domain polymer sustained alignment (4D-PSA) mode, or a fish bonepolymer sustained alignment (FB-PSA) mode, vertical alignment films arepreferably used. In the case of vertical alignment films, it ispreferable that the pre-tilt angle is substantially 90°. Particularly,in the case of the 4D-RTN mode or the 4D-PSA mode, it is preferable thatthe pre-tilt angle is set to 85.0° to 89.7°.

The alignment films 22 are photo-alignment films formed from a materialthat exhibits a photo-alignment characteristic. A material that exhibitsa photo-alignment characteristic corresponds in general to a materialwhose structure is changed by being irradiated with light(electromagnetic wave) such as ultraviolet light or visible light todevelop a nature (alignment regulating power) of regulating thealignment of liquid crystal molecules present in the vicinity thereof orto change in size and/or direction of the alignment regulating power.

Examples of the material that exhibits a photo-alignment characteristicinclude a material including a photoreactive part that causes a reactionsuch as dimerization (formation of dimer), isomerization, photo-Friesrearrangement, or degradation in response to irradiation with light. Asa photoreactive part (functional group) that dimerizes or isomerizes inresponse to irradiation with light, for example, cinnamate shown in aformula (B-1) below, 4-chalcone shown in a formula (B-2-1) below,4′-chalcone shown in a formula (B-2-2) below, coumarin shown in aformula (B-3) below, or stillbene shown in a formula (B-4) below ispreferably used. An isomerization reaction and a dimerization reactionof the cinnamate are shown in a formula (B-1-I) below.

In addition, as a photoreactive part (functional group) that isomerizesin response to irradiation with light, for example, azobenzene ispreferably used. Trans-azobenzene is shown in a formula (B-5-1) below,and cis-azobenzene is shown in a formula (B-5-2) below.

As a photoreactive part that causes photo-Fries rearrangement, forexample, a phenol ester structure shown in a formula (B-6) below ispreferably used. The phenol ester structure causes photo-Friesrearrangement as shown in a formula (B-6-I) below.

As a photo reactive part that is decomposed by being irradiated withlight, for example, a cyclobutane structure is preferably used. Examplesof a photo-alignment film including a cyclobutane structure includes apolymer obtained by subjecting an acid anhydride including a cyclobutanestructure shown in a formula (B-7-1) below and a diamine compound shownin a formula (B-7-2) below serving as monomers to copolymerization. Itis to be noted that, any of hydrogen atoms in the cyclobutane structureshown in the formula (B-7-1) below may be substituted by other atoms orfunctional groups.

When the acid anhydride including the cyclobutane structure shown in theformula (B-7-1) above reacts with the diamine compound shown in theformula (B-7-2) above, polyamic acid is produced as shown in a formula(B-7-I) below. When a photo-alignment film is heated, polyimide isproduced as shown in a formula (B-7-II) below. In an obtainedphoto-alignment film, a polyamic acid unit shown in the formula (B-7-I)below and a polyimide unit shown in the formula (B-7-II) are present atthe same time. Further, when being irradiated with light, the ring ofthe cyclobutane structure of the polymer shown in the formula (B-7-II)below opens as shown in a formula (B-7-III), and exhibits aphoto-alignment characteristic.

The alignment films 22 contain carboxyl groups on the liquid crystallayer side. The alignment films 22 are obtained by, for example,performing condensation polymerization of polyamic acid, the acidanhydride including the cyclobutane structure shown in the formula(B-7-1) above, and the diamine shown in the formula (B-7-2). When thealignment films are formed by performing condensation polymerization ofthese materials, carboxyl groups are exposed on the liquid crystal layerside of the alignment films because a polyamic acid unit and a polyimideunit are present at the same time. While the carboxyl groups have a highaffinity with components such as epoxy resin and a silane coupling agentin the seal material and increases the adhesion strength between thealignment films and the seal material, in the case where a compound thatreacts with acid is contained in the liquid crystal composition, thereis a risk that the compound reacts with the carboxyl groups and ionicimpurities and radicals are generated. However, the nitrobenzenederivative described above does not react with acid such as a carboxylgroup, and thus the nitrobenzene derivative is not ionized and canscavenge the radicals.

In addition, in the present embodiment, a polymer sustained alignment(PSA) technique may be used. In the PSA technique, a liquid crystalcomposition containing a photopolymerizable monomer is sealed betweenthe pair of substrates 21, followed by irradiation of the liquid crystallayer 23 with light to polymerize the photopolymerizable monomer so thata polymer is formed on the surfaces of the alignment films 22. The PSAtechnique utilizes the polymer to fix the pre-tilt of the liquidcrystal.

Examples of application of the PSA technique includes an embodiment thatincludes a layer containing a polymer obtained by polymerizing aphotopolymerizable monomer represented by a formula (C) below on theliquid crystal layer 23 side surfaces of the alignment films 22.A1-Y-A2  (C)(In the formula, Y represents a structure including at least one benzenering and/or condensed benzene ring; any of hydrogen atoms in the benzenering and condensed benzene ring may be substituted by a halogen atom; atleast one of A1 and A2 represents acrylate or methacrylate; and A1 andA2 are directly bonded to the benzene ring or the condensed benzenering.)

It is preferable that a skeleton Y in the formula (C) above has astructure represented by a formula (C-1), (C-2), or (C-3) below. It isto be noted that, each hydrogen atom in the formulae (C-1), (C-2), and(C-3) may be independently substituted by a halogen atom.

Specific examples of the photopolymerizable monomer represented by theformula (C) above include formulae (C-1-1), (C-1-2), and (C-3-1).

In the present embodiment, the liquid crystal layer 23 contains a liquidcrystal material and the nitrobenzene derivative.

<Liquid Crystal Material>

An anisotropy of dielectric constant (Δε) of the liquid crystal materialdefined by a formula (P) below may be a negative value or a positivevalue. That is, the liquid crystal material may have a negativeanisotropy of dielectric constant and may have a positive anisotropy ofdielectric constant. As the liquid crystal material having a negativeanisotropy of dielectric constant, for example, a liquid crystalmaterial having Δε of −1 to −20 can be used. As the liquid crystalmaterial having a positive anisotropy of dielectric constant, forexample, a liquid crystal material having Δε of 1 to 20 can be used.Δε=(dielectric constant in a major axis direction)−(dielectric constantin a minor axis direction)  (P)

In a conventional liquid crystal display device to which a radicalscavenger is not added, there is a tendency that the malfunctions ofimage sticking and stain appear more prominently when a liquid crystalmaterial having a negative anisotropy of dielectric constant is usedthan when a liquid crystal material having a positive anisotropy ofdielectric constant. It is assumed that this is because a liquid crystalmaterial having a negative anisotropy of dielectric constant has largepolarization in the minor axis direction and the influence of decreaseof the VHR caused when being ionized becomes greater. That is, thenitrobenzene derivative used in the present invention has a great effectin a system in which a liquid crystal material having a negativeanisotropy of dielectric constant and a photo-alignment film are used incombination.

It is preferable that at least one component of the liquid crystalmaterial is a compound including an alkenyl structure. Examples of thecompound including an alkenyl structure include compounds represented bya formula (D-1), (D-2), or (D-3) below.

(In the formulae, m and n represent the same or different integers, andare preferably 1 to 6.)

Specific examples of the compounds including an alkenyl structurerepresented by the formula (D-1) above include a compound represented bya formula (D-1-1) below.

A compound including an alkenyl structure can reduce the viscosity ofthe liquid crystal material. Therefore, at least one component of theliquid crystal material being a compound including an alkenyl structurecan improve the response speed in both cases of the liquid crystalmaterial having a positive anisotropy of dielectric constant and anegative anisotropy of dielectric constant. On the other hand, a doublebond of an alkenyl part is likely to be attacked by a radical, and thusit is considered that the alkenyl part is likely to be a factor of thedecrease of VHR particularly when used in combination with aphoto-alignment film.

It is preferable that at least one component of the liquid crystalmaterial is a compound including an alkoxy structure. Examples of thecompound including an alkoxy structure include compounds represented bya formula (E-1), (E-2), (E-3), (E-4), or (E-5) below.

(In the formulae, m and n represent the same or different integers, andare preferably 1 to 7.)

Specific examples of the compounds including an alkoxy structurerepresented by the formula (E-3) above include a compound represented bya formula (E-3-1) below.

<Nitrobenzene Derivative>

A nitrobenzene derivative functions as a radical scavenger. Thenitrobenzene derivative efficiently reacts with alignment film radicalsgenerated in the photo-alignment films and liquid crystal radicalsgenerated by the alignment film radicals transferring to the liquidcrystal, and deactivates the alignment film radicals and liquid crystalradicals. In addition, the nitrobenzene derivatives can scavengeradicals without generating ionic impurities with members such as analignment film. Further, moisture coming from the outside can betrapped, and thus neutral impurities being ionized by the moisturecoming from the outside of the liquid crystal panel can be suppressed.According to these effects, the decrease of VHR of the liquid crystaldisplay device can be suppressed and occurrence of stain and imagesticking of a screen can be prevented.

FIG. 2 is a diagram illustrating a reaction mechanism of a nitrobenzenederivative deactivating a radical generated in a photo-alignment film.As shown in a formula (A-I) of FIG. 2, a photofunctional group P_(AL) ina photo-alignment film is excited by being irradiated with light (amountof energy: hv), and alignment film radicals R_(AL) are generated. Asshown in a formula (A-II) of FIG. 2, a nitrobenzene derivative added tothe liquid crystal material constituting the liquid crystal layer 23 canselectively react with an alignment film radical R_(AL) and deactivatethe alignment film radical R_(AL). However, the nitrobenzene derivativeitself generates a new radical (—N—O⋅) as a result of a reaction betweena nitro group and the alignment film radical R_(AL). As shown in aformula (A-III) of FIG. 2, the new radical (—N—O⋅) bonds to anotheralignment film radical R_(AL) and thus both of the radical generatedfrom the nitrobenzene derivative and the alignment film radical R_(AL)are eliminated. A nitrobenzene derivative can scavenge two radicals byone nitrobenzene skeleton, and thus has a high radical scavengingefficiency. As described above, in a system to which a nitrobenzenederivative has been added, reactions progress in the order of formula(A-I)→formula (A-II)→formula (A-III). The reactions of the formulae(A-II) and (A-III) are reversible reactions, and cycles of scavengingand releasing radicals are repeated. Thus, ionization of radicals can behindered. As a result of this, generated radicals can be continuouslydeactivated without decreasing the amount of the nitrobenzenederivative, and thus generation of ions by the radicals can becontinuously hindered for a long period of time. That is, by using anitrobenzene derivative, the decrease of VHR caused by exposure to lightfrom the backlight can be suppressed for a long period of time byaddition of just a small amount.

FIG. 3 is a diagram illustrating a reaction of a nitrobenzene derivativewith a water molecule. As illustrated in FIG. 3, a nitrobenzenederivative can form hydrogen bonds between two oxygen atoms included ina nitro group and two hydrogen atoms included in a water molecule.Accordingly, water coming from the outside can be trapped, and thusstain in the periphery of the liquid crystal panel can be prevented evenwhen the width of the seal material is reduced.

In addition, a nitrobenzene derivative has a high reactivity withradicals generated in the photo-alignment films, and thus can quicklydeactivate radicals in the liquid crystal layer 23. Therefore, in thecase of using an antioxidant in combination, this antioxidant beingconsumed by reacting with radicals generated in the photo-alignmentfilms can be effectively suppressed, and, as a result, generation ofoxides in the liquid crystal layer can be also suppressed. Therefore,image sticking and stain derived from oxides can be also prevented.

Further, as an effect of greatly suppressing generation of ions in theliquid crystal layer 23 by using a nitrobenzene derivative, it becomespossible to set a long frame period at the time of driving the liquidcrystal display device. That is, it becomes possible to drive the liquidcrystal display device at a low frequency, and, as a result, it becomespossible to reduce consumption of electricity.

A nitrobenzene derivative is a compound represented by a formula (F)below.

(In the formula, X represents a monovalent organic group.)

The nitrobenzene derivative represented by the formula (F) above may bea compound including two nitrobenzene skeletons in one molecule, andexamples thereof include compounds represented by a formula (G-1),(G-2), (G-3), (G-4), or (G-5) below.

(In the formulae, m represents an even number from 2 to 18.)

Specific examples of the nitrobenzene derivative including onenitrobenzene skeleton in one molecule represented by the formula (G-1)above include a compound represented by a formula (G-1-1) below.

The nitrobenzene derivative represented by the formula (F) above may bea compound including one nitrobenzene skeleton in one molecule, andexamples thereof include compounds represented by a formula (H-1),(H-2), (H-3), or (H-4) below.

(In the formulae, n represents an integer from 1 to 18.)

Specific examples of the nitrobenzene derivative including onenitrobenzene skeleton in one molecule represented by the formula (H-1)above Include a compound represented by a formula (H-1-1) below.

The concentration of the nitrobenzene derivative is preferably 1 ppm orhigher and 5000 ppm or lower. Within this range, radicals generated inthe photo-alignment films can be sufficiently deactivated, and theeffect of suppressing the decrease of VHR can be achieved particularlysufficiently. A nitrobenzene derivative can be present stably, and thusthe concentration of the nitrobenzene derivative being too high may notbe desirable for the liquid crystal alignment characteristic. Therefore,an antioxidant that will be described later may be added instead ofincreasing the concentration of the nitrobenzene derivative. A morepreferable upper limit of the concentration of the nitrobenzenederivative is 500 ppm, and a furthermore preferable upper limit thereofis 250 ppm.

<Antioxidant>

The liquid crystal layer 23 may further contain a phenolic antioxidantrepresented by a formula (I) below. In addition, the liquid crystallayer 23 may contain an antioxidant different from a phenolicantioxidant.

(In the formula, X represents a monovalent organic group.)

FIG. 4 is a diagram illustrating an effect of a phenolic antioxidant inthe present invention. In FIG. 4, R represents a group such as an alkylgroup contained in the liquid crystal material, alignment film, and sealmaterial. As shown in a formula (J-I) of FIG. 4, when oxygen enters theliquid crystal panel and light or heat energy is applied, the alkylgroup (R), for example, contained in the liquid crystal material,alignment films, and seal material is oxidized, and an oxidized matter(ROOH) is generated. This oxidized matter generates radicals, and theradicals are ionized in a condition in which no antioxidant or radicalscavenger is present. In the case where the liquid crystal material Isoxidized and ionized, ions are generated in the liquid crystal layer 23.In addition to this, ions are generated in the liquid crystal layer 23also in the case where the alignment films or the seal agent is oxidizedbecause oxidized matter dissociated from a polymer constituting thealignment films or the seal agent is ionized and eluted into the liquidcrystal layer 23. Accordingly, ions in the liquid crystal layer 23causes the decrease of VHR. Meanwhile, as a result of adding anantioxidant, as shown in formulae (J-II) and (J-III) of FIG. 4, theantioxidant reacts with radicals before the radicals are ionized, andthus generation of ions caused by oxidation of the liquid crystalmaterial, photo-alignment films, and seal material can be prevented, inaddition, the amount of the antioxidant does not decrease in the cycleshown in the formulae (J-II) and (J-III) of FIG. 4, and thus ionizationof the radicals can be prevented for a long period of time.

As illustrated in FIG. 4, an antioxidant has a function of causingdissociation of oxygen from oxide (reduction) by repeating a cycle ofdissociation→addition→dissociation of hydrogen groups, and suppressesdeterioration (decomposition and ionization) caused by oxidation for along period of time. However, the antioxidant is sometimes consumed by areaction of the antioxidant with the photo-alignment films. FIG. 5 is adiagram illustrating a reaction of a phenolic antioxidant with aphoto-alignment film. As shown in a formula (K-I) of FIG. 5, when aphoto-alignment film is irradiated with ultraviolet light from thebacklight, a cinnamate group, which is a photofunctional group, cleavesand radicals (—CO⋅ and —O⋅) are generated. Next, as shown in a formula(K-II) of FIG. 5, the generated radical reacts with the antioxidant, andthe antioxidant itself turns into an antioxidant radical. Here, as shownin a formula (K-III) of FIG. 5, the antioxidant radical sometimes bondsto a radical on the photo-alignment film side generated by the cleavageof the cinnamate group. In the case of the formula (K-III), theantioxidant bonded to the radical of a chain on the photo-alignment filmside cannot return to the antioxidant again, and thus the amount of theantioxidant in the liquid crystal layer 23 gradually decreases. Asdescribed above, in the case where the consumption of the antioxidantcontinues for a long period of time, there is a possibility thatprevention of the oxidation of the liquid crystal layer 23 andphoto-alignment films becomes insufficient. Although the case where acarbon atom is on the main chain side of the alignment film (—COO—) isshown in the formula (K-I) of FIG. 5, the same applies to the case wherean oxygen atom is on the main chain side of the alignment film (—OCO—).In addition, although an example of a cinnamate group is illustrated inFIG. 5, it is known that consumption of the antioxidant occurs similarlyin the case of using another photofunctional group such as an azobenzenegroup. To address this, in the present embodiment, the consumption of anantioxidant is prevented by using the antioxidant in combination with anitrobenzene derivative. A radical scavenger has a function ofscavenging radicals generated by both oxides and non-oxides in thealignment films and liquid crystal, and prevents ionization of theradicals by repeating scavenging and releasing the radicals. It isconsidered that, with a nitrobenzene derivative that has a higherreactivity with radicals than an antioxidant does and continuouslyscavenges radicals in the photo-alignment films and liquid crystal, thereactions of consuming the antioxidant is suppressed, and thus thefunction of preventing oxidation can be retained.

Specific examples of the antioxidant represented by the formula (I)above include antioxidants represented by a formula (I-1), (I-2), or(I-3) below, and more specifically a formula (I-1-1) below.

(In the formula, n represents an integer, and is preferably 3 to 20.)

Other specific examples of the phenolic antioxidant represented by theformula (I) above include compounds represented by a formula (I-a),(I-b), (I-c), (I-d), (I-e), or (I-f) below.

It is preferable that the concentration of the antioxidant is 1 ppm orhigher and 10 weight % or lower. In this range, oxygen entering theliquid crystal panel from the outside can be prevented from oxidizingthe liquid crystal material, and thus the image sticking and stain ofthe display caused by oxides can be effectively prevented. In addition,part of the radicals generated in the photo-alignment films can bedeactivated by using an antioxidant as well as by using a nitrobenzenederivative, and the effect of suppressing the decrease of VHR can beparticularly sufficiently achieved. A more preferable lower limit of theconcentration is 10 ppm, a more preferable upper limit thereof is 5weight %, and a furthermore preferable upper limit thereof is 1 weight%.

The alignment mode of the liquid crystal panel described above is notparticularly limited, and, for example, horizontal alignment modes suchas the FFS mode and the IPS mode; vertical alignment modes such as the4D-RTN mode, the 4D-PSA mode, and the FS-PSA mode; and the TN mode canbe used.

In the case where the alignment mode of the liquid crystal paneldescribed above is a horizontal alignment mode, radicals are likely tobe generated in the photo-alignment films, and thus the effect of addinga nitrobenzene derivative can be achieved more prominently. That is,whereas just slightly changing the pre-tilt angle from 90° is sufficientin photo-alignment treatment (radiation of polarized UV light) in avertical alignment mode, the azimuth (direction in the substrate plane)of the alignment of the liquid crystal needs to be controlled moreprecisely in photo-alignment treatment in a horizontal alignment mode.Therefore, the amount of radiation in the photo-alignment treatment inthe horizontal alignment mode is normally larger than in the case of thevertical alignment mode by one digit or more, and more radicals are morelikely to be generated by a side reaction than in the case of thevertical alignment mode. A nitrobenzene derivative contained in theliquid crystal layer can deactivate radicals generated at the time ofthe photo-alignment treatment, and thus the radicals can be effectivelyprevented from remaining after completion of the liquid crystal panel(after injection of liquid crystal).

In the FFS mode, a structure (FFS electrode structure) including aplanar electrode, a slit electrode, and an insulating film disposedbetween the planar electrode and the slit electrode is provided in atleast one of the substrates 21, and an oblique electric field (fringeelectric field) is formed in the liquid crystal layer 23 adjacent to thesubstrates 21. Normally, the slit electrode, insulating film, and planarelectrode are arranged in this order from the liquid crystal layer 23side. The slit electrode can be, for example, an electrode including alinear opening portion surrounded all around by an electrode as a slitor a comb-shaped electrode including plural comb tooth portions andlinear cutouts that are positioned between the comb tooth portions andconstitute slits.

In the IPS mode, at least one of the substrates 21 is provided with apair of comb-shaped electrodes, and a transverse electric field isformed in the liquid crystal layer 23 adjacent to the substrate 21. Asthe pair of comb-shaped electrodes, for example, a pair of electrodeseach including plural comb tooth portions and arranged such that thecomb tooth portions engage with one another can be used.

In the 4D-RTN mode, one of the substrates 21 is provided with a pixelelectrodes, and the other of the substrates 21 is provided with a commonelectrode, so that a longitudinal electric field is formed in the liquidcrystal layer 23 adjacent to the substrates 21. The pair of substrates21 includes a vertical alignment film, and is arranged such thatalignment treatment directions are perpendicular to each other. In the4D-RTN mode, four alignment directions different from one another can bedefined in one pixel, and thus an excellent viewing angle characteristiccan be achieved. Meanwhile, the pre-tilt angle needs to be controlledwith a high precision, and thus photo-alignment treatment is preferablyused.

In the liquid crystal panel 20 of the present embodiment, normally, thepair of substrates 21 are attached together by the seal material (notillustrated) provided so as to surround the periphery of the liquidcrystal layer 23, and the liquid crystal layer 23 is retained in apredetermined region. For example, an epoxy resin or the like materialcontaining inorganic filler or organic filler and a curing agent can beused as the seal material, and may further contain a silane couplingagent.

In addition, a polarizing plate (linear polarizer) may be disposed oneach side of the pair of substrates 21 opposite to the liquid crystallayer 23. Typical examples of the polarizing plate Include a polyvinylalcohol (PVA) film on which dichroic anisotropic material such as aniodine complex is adsorbed and aligned. Normally, a laminate of a PVAfilm and protective films such as triacetyl cellulose films disposed onboth surfaces of the PVA film is put into practical use. In addition,optical films such as phase difference films may be disposed betweenpolarizing plates and the pair of substrates 21.

As illustrated in FIG. 1, in the liquid crystal display device of thepresent embodiment, the backlight 10 is disposed on the back surfaceside of the liquid crystal panel. A liquid crystal display device havingsuch a configuration is typically referred to as a transmissive liquidcrystal display device. The backlight 10 is not particularly limited aslong as the backlight 10 emits light including visible light, and mayemit light including only visible light or light including both ofvisible light and ultraviolet light. For enabling color display of theliquid crystal display device, a backlight 10 that emits white light ispreferably used. With regard to the type of the backlight 10, forexample, light emitting diodes (LEDs) are preferably used. It is to benoted that, in the present description, “visible light” is used to referto light (electromagnetic wave) having a wavelength of 380 nm or largerand smaller than 800 nm.

A feature of the present invention is also in deactivating, by using anitrobenzene derivative, radicals generated in the photo-alignment filmsas a result of exposure to light from the backlight 10. Therefore, thenitrobenzene derivative can function effectively in the case where atleast part of a light emission spectrum of the backlight 10 overlaps atleast part of an absorption spectrum of the photo-alignment films.

The liquid crystal display device of the present embodiment isconstituted by plural members including an external circuit such as atape carrier package (TCP) or a print circuit board (PCB); an opticalfilm such as a viewing angle expansion film or a luminance enhancingfilm; and a bezel (frame) in addition to the liquid crystal panel 20 andthe backlight 10, and a member may be included in another memberdepending on the member. Since members other than the members havingbeen already described are not particularly limited and members normallyused in the field of liquid crystal display device can be used, thedescription thereof will be omitted.

An embodiment of the present invention has been described above, and allindividual matters described above can be applied to the presentinvention overall.

Although the present invention will be described in further details byusing examples and comparative examples below, the present invention isnot limited to the examples.

EXAMPLE 1

A liquid crystal display device including a liquid crystal panel of a4-domain reverse TN mode (4D-RTN mode) was actually produced by thefollowing method.

First, a TFT substrate including TFTs, pixel electrodes, and so forth,and a CF substrate including a black matrix, color filters, a commonelectrode, and so forth were prepared. Then, an alignment film solutionwas applied to the surface of each of the TFT substrate and the CFsubstrate. A solid component of the alignment film solution was apolymer material including a polysiloxane structure as a main skeletonand cinnamate groups represented by a formula (B-1) below serving asphotofunctional groups as aide chains, and a polyamic acid.

Next, both the substrates were heated at 80° C. to volatilize thesolvent in the alignment film solution. Subsequently, both thesubstrates were heated at 230° C. for post-baking. In the post-baking,imidization (dehydration cyclization reaction) occurred in part of apolyamic acid structure, and a polyimide structure was formed. Then, asphoto-alignment treatment, the surfaces of both the substrates wereirradiated with linearly polarized light having a main wavelength of 313nm at an intensity of 22 mJ/cm². The direction of polarization of thelinearly polarized light was set to be inclined by 40° with respect to asurface on which the alignment film solution was applied. As a result ofthe irradiation with the linearly polarized light, the cinnamate groupsunderwent an isomerization reaction and a dimerization reaction, andthus an alignment regulating power developed. In this way, verticalalignment films in which a sufficient alignment regulating power wasdeveloped by the irradiation with light were obtained. The filmthickness after the post-baking was 100 nm. The obtained alignment filmseach had a two-layer structure in which a polymer layer of a polymerizedpolyamic acid was formed on the substrate side and a polymer layer of apolymerized polymer material including a polysiloxane structure as amain skeleton and cinnamate groups represented by the formula (B-1)above serving as photofunctional groups as side chains was formedthereon.

Next, a liquid crystal composition was dripped onto the TFT substrate,and a seal material curable by heat and/or visible light was poured onthe CF substrate by a dispenser. Then, the TFT substrate and the CFsubstrate were attached together such that the alignment treatmentdirections thereof were perpendicular to each other, and the liquidcrystal composition was sealed between the substrates. The alignmentfilms described above contained carboxyl groups on the liquid crystallayer side because the polyamic acid was partially exposed on thesurface. At the time of attaching the substrates together, a displayregion was shielded from light, and exposure for curing the sealmaterial was performed. The width of the seal material was 1.0 mm.

A liquid crystal material containing a compound including an alkenylstructure represented by a formula (D-1-1) below to which a nitrobenzenederivative represented by a formula (G-1-1) below was added was used asthe liquid crystal composition. The concentration of the nitrobenzenederivative was set to 200 ppm with respect to the total amount of theliquid crystal composition. The liquid crystal material had a negativeanisotropy of dielectric constant (Δε=−3.5).

Then, a pair of polarizing plates were attached to the back surface(surface on which light from a backlight is incident) side of the TFTsubstrate and the observation surface (surface from which light from abacklight is emitted) aide of the CF substrate such that thepolarization axes were in a relationship of crossed Nicols, and thus aliquid crystal panel for the 4D-RTN mode was produced.

Next, a backlight including white LEDs was attached to the back surfaceside of the liquid crystal panel, and thus the liquid crystal displaydevice of Example 1 was completed.

COMPARATIVE EXAMPLE 1

A liquid crystal panel for the 4D-RTN mode was produced in the samemanner as in Example 1 except that no nitrobenzene derivative was addedto the liquid crystal composition.

COMPARATIVE EXAMPLE 2

A liquid crystal panel for the 4D-RTN mode was produced in the samemanner as in Example 1 except that rubbing alignment films, whichdevelop an alignment regulating power by rubbing treatment, were formedinstead of the photo-alignment films, which develop an alignmentregulating power by irradiation with light, and that no nitrobenzenederivative was added to the liquid crystal composition.

In Comparative Example 2, a polymer material including a polyamic acidstructure in the main chain and not including a photoreactive part wasused as the solid component of the alignment film solution. In addition,rubbing treatment was performed without performing photo-alignmenttreatment and an alignment regulating power perpendicular to the surfaceof a substrate was developed.

(Assessment Test 1)

A current was supplied, for 1000 hours, to the liquid crystal panelsproduced in Example 1 and Comparative Examples 1 and 2 in a state wherethe backlights (luminance of 8000 cd/m²) were on. At this time, a blackbackground and a white box pattern were displayed on the screens of theliquid crystal display devices. The VHR before the supply of current(initial VHR) and after 1000 hours was measured for each liquid crystalpanel. In addition, after the supply of current for 1000 hours, theentire screen was switched to display at a gray scale value of 64, thescreen of each liquid crystal panel was visually observed, and presenceof stain and image sticking was checked. The results are shown in Table1.

TABLE 1 VHR (%) Stain Image sticking VHR (%) (After (After (After(Initial) 1000 hours) 1000 hours) 1000 hours) Example 1 99.0 98.0 Notpresent Not present Comparative 99.0 96.0 Present Present Example 1Comparative 99.0 98.0 Not present Slightly present Example 2

As can be seen from Table 1, the result of comparison between Example 1and Comparative Example 1 indicates that the decrease of VHR wassuccessfully suppressed by adding a nitrobenzene derivative to theliquid crystal material. In addition, the result of comparison betweenComparative Example 1 and Comparative Example 2 indicates that theprominent decrease of VHR occurred in Comparative Example 1 was causedby the photo-alignment films. That is, it was revealed that the effectobserved in Example 1 is prominent when a nitrobenzene derivative andphoto-alignment films are used in combination. In addition, screen stainand image sticking were observed in Comparative Example 1. InComparative Example 2, although no stain was observed, slight imagesticking was observed. In contrast, in Example 1 in which a nitrobenzenederivative was added to the liquid crystal material, no stain or imagesticking was observed.

In addition, a liquid crystal component including an alkenyl structureis effective for reducing the viscosity of the liquid crystal material.Meanwhile, a double bond included in the alkenyl structure is likely tobe attached by a radical, and thus is likely to be a factor of thedecrease of VHR in the case of being used in combination withphoto-alignment films that can be a generation source of the radical. InExample 1, the attack of a radical on the alkenyl structure can beeffectively prevented by adding a nitrobenzene derivative to the liquidcrystal material. It is to be noted that the liquid crystal componentincluding an alkenyl structure is preferably added not only to a liquidcrystal material having a negative anisotropy of dielectric constant butalso to & liquid crystal material having a positive anisotropy ofdielectric constant from the viewpoint of improving the response speedof the liquid crystal display device.

EXAMPLE 2

A liquid crystal panel for the 4D-RTN mode was produced in the samemanner as in Example 1 except that no compound including an alkenylstructure was added to the liquid crystal composition.

COMPARATIVE EXAMPLE 3

A liquid crystal panel for the 4D-RTN mode was produced in the samemanner as in Example 2 except that no liquid crystal material containinga nitrobenzene derivative was added to the liquid crystal composition.

(Assessment Test 2)

A current was supplied, for 1000 hours, to the liquid crystal panelsproduced in Example 2 and Comparative Example 3 in the same manner as inAssessment Test 1, and the VHR before the supply of current (initialVHR) and after 1000 hours was measured. In addition, the screen of eachliquid crystal panel was visually observed, and presence of stain andimage sticking was checked. The results are shown in Table 2.

TABLE 2 VHR (%) Stain Image sticking VHR (%) (After (After (After(Initial) 1000 hours) 1000 hours) 1000 hours) Example 2 99.5 99.5 Notpresent Not present Comparative 99.5 98.3 Not present Slightly presentExample 3

As can be seen from Table 2, the result of comparison between Example 2and Comparative Example 3 indicates that the decrease of VHR wassuccessfully suppressed by adding a nitrobenzene derivative to theliquid crystal material. In addition, the result of comparison betweenExample 2 and Comparative Example 3 Indicates that the decrease of VHRwas successfully suppressed by adding a nitrobenzene derivative evenwithout adding a liquid crystal material containing a compound includingan alkenyl structure to the liquid crystal composition. In addition, inComparative Example 3, although no stain was observed, slight imagesticking was observed. In contrast, in Example 2 in which a nitrobenzenederivative was added to the liquid crystal material, no stain or imagesticking was observed.

EXAMPLE 3

A liquid crystal panel of the fringe field switching mode (FFS mode) wasactually produced by the following method.

First, a TFT substrate including TFTs, an FFS electrode structure, andso forth, and a color filter substrate (CF substrate) including a blackmatrix, color filters, and so forth were prepared. Then, an alignmentfilm solution was applied to the surface of each of the TFT substrateand the CF substrate. A solid component of the alignment film solutionwas a polymer material including a polyamic acid structure and anazobenzene structure having a photo-alignment characteristic in the mainchain.

Next, both the substrates were heated at 70° C. to volatilize thesolvent in the alignment film solution. Subsequently, as photo-alignmenttreatment, the surfaces of both the substrates were irradiated withlinearly polarized light having a main wavelength of 365 nm at anintensity of 2000 mJ/cm². The polarization direction of the linearlypolarized light was set so as to be perpendicular to the direction inwhich the liquid crystal was to be aligned. As a result of theirradiation with the linearly polarized light, a trans-cis isomerizationreaction occurred at the azobenzene structure, and thus an alignmentregulating power developed. A trans-azobenzene structure has a structurerepresented by a formula (B-5-1) below, and a cis-azobenzene structurehas a structure represented by a formula (B-5-2) below.

Then, both the substrates were heated at 220° C. for post-baking. In thepost-baking, imidization (dehydration cyclization reaction) occurred inpart of a polyamic acid structure, and a polyimide structure was formed.In this way, horizontal alignment films in which a sufficient alignmentregulating power was developed by the irradiation with light wereobtained. The film thickness after the post-baking was 100 nm.

Subsequently, the TFT substrate and the CF substrate were attachedtogether in the same manner as in Example 1, and the liquid crystalcomposition was sealed between the substrates. The same liquid crystalcomposition as in Example 1 was used. The alignment films describedabove contained carboxyl groups on the liquid crystal layer side becausethe polyamic acid that was not imidized was partially exposed on thesurface. The width of the seal material was 0.8 mm.

Then, a pair of polarizing plates were attached to the back surface(surface on which light from a backlight is incident) side of the TFTsubstrate and the observation surface (surface from which light from abacklight is emitted) side of the CF substrate such that thepolarization axes were in a relationship of crossed Nicols. As describedabove, the liquid crystal panel for the FFS mode was produced.Subsequently, a backlight including white LEDs was attached to the backsurface side of the liquid crystal panel, and thus the liquid crystaldisplay device of Example 3 was completed.

COMPARATIVE EXAMPLE 4

A liquid crystal panel for the FFS mode was produced in the same manneras in Example 3 except that no nitrobenzene derivative was added to theliquid crystal composition.

COMPARATIVE EXAMPLE 5

A liquid crystal panel for the FFS mode was produced in the same manneras in Example 3 except that rubbing alignment films, which develop analignment regulating power by rubbing treatment, were formed instead ofthe photo-alignment films, which develop an alignment regulating powerby irradiation with light, and that no nitrobenzene derivative was addedto the liquid crystal composition. In Comparative Example 5, a polymermaterial including a polyamic acid structure in the main chain was usedas the solid component of the alignment film solution. In addition,rubbing treatment was performed without performing photo-alignmenttreatment.

(Assessment Test 3)

A current was supplied, for 1000 hours, to the liquid crystal panelsproduced in Example 3 and Comparative Examples 4 and 5 in the samemanner as in Assessment Test 1, and the VHR before the supply of current(initial VHR) and after 1000 hours was measured. In addition, the screenof each liquid crystal panel was visually observed, and presence ofstain and image sticking was checked. The results are shown in Table 3.

TABLE 3 VHR (%) Stain Image sticking VHR (%) (After (After (After(Initial) 1000 hours) 1000 hours) 1000 hours) Example 3 99.0 98.3 Notpresent Not present Comparative 99.0 96.0 Present Present Example 4Comparative 99.0 98.0 Not present Slightly present Example 5

As can be seen from Table 3, the result of comparison between Example 3and Comparative Example 4 indicates that the decrease of VHR wassuccessfully suppressed by adding a nitrobenzene derivative to theliquid crystal material. In addition, the result of comparison betweenComparative Example 4 and Comparative Example 5 indicates that theprominent decrease of VHR occurred in Comparative Example 4 was causedby the photo-alignment films. That is, it was revealed that the effectobserved in Example 3 is prominent when a nitrobenzene derivative andphoto-alignment films are used in combination. In addition, a screenstain and image sticking were observed in Comparative Example 4. InComparative Example 5, although no stain was observed, slight imagesticking was observed. In contrast, in Example 3 in which a nitrobenzenederivative was added to the liquid crystal material, no stain or imagesticking was observed.

The reason why the decrease of VHR is caused by photo-alignment films isconsidered as follows.

The azobenzene structure included in the photo-alignment films used inExample 3 and Comparative Example 4 is subjected to alignment treatmentby light having a wavelength of 365 nm, which is close to a visiblelight region. Meanwhile, the backlight of the liquid crystal displaydevice mainly emits light of a visible light region for color display,and, from the results of Comparative Example 4, it is assumed that theshort wavelength side of the light emission spectrum of the backlightand the long wavelength side of the absorption spectrum of theazobenzene structure slightly, although at a level difficult to detectin an actual spectrum analysis, overlap each other and radicals aregenerated. For example, as shown in a reaction formula shown below, theazobenzene structure is considered to undergo a photo-cleavage reactionbeing caused by light from the backlight. With regard to this, from theresults of Example 3, it was revealed that the nitrobenzene derivativeeffectively deactivates the radicals generated by the reaction of thephoto-alignment films and thus the decrease of VHR can be prevented.

Examples of a photoreactive part that is subjected to alignmenttreatment by light having a wavelength close to a visible light regionsimilarly to the azobenzene structure include cinnamate, chalcone,coumarin, stillbene, and phenol ester. These photoreactive parts are allconsidered to absorb, although slightly, light having a wavelength of340 nm or larger, and thus can absorb the light from the backlight andbe a generation source of radicals similarly to the azobenzenestructure. For example, in the case of cinnamate, chalcone, or phenolester, photo-Fries rearrangement (cleavage of an ester group) occurs anda radical is generated, and, in the case of chalcone, as shown in areaction formula shown below, abstraction of hydrogen or photo-cleavageoccurs and a radical is generated. Therefore, it is preferable to add anitrobenzene derivative to the liquid crystal material also in the caseof using photo-alignment films including these photoreactive parts.

EXAMPLE 4

A liquid crystal display device including a liquid crystal panel of theFFS mode was actually produced in the same manner as in Example 3. Thesolid component of the alignment film solution was a polymer materialincluding a polyamic acid structure obtained by performing condensationpolymerization of an acid anhydride represented by a formula (B-7-1)below and a diamine compound represented by a formula (B-7-2) below. Itis to be noted that any of hydrogen atoms of the cyclobutane in the acidanhydride represented by the formula (B-7-1) below may be substituted byother atoms or functional groups.

Next, both the substrates were heated at 70° C. to volatilize thesolvent in the alignment film solution. Subsequently, both thesubstrates were heated at 230° C. for post-baking. In the post-baking,imidization (dehydration cyclization reaction) occurred in part of apolyamic acid structure, and a polyimide structure was formed. Then, asphoto-alignment treatment, the surfaces of both the substrate wereirradiated with linearly polarized light having a main wavelength of 254nm at an intensity of 600 mJ/cm². The polarization direction of thelinearly polarized light was set so as to be perpendicular to thedirection in which the liquid crystal was to be aligned. As a result ofthe irradiation with the linearly polarized light, a decompositionreaction in which the cyclobutane part cleaves occurs as shown in aformula (B-7-A) below, the alignment regulating power in the polymerchain direction was eliminated, and thus an alignment regulating powerin a direction perpendicular thereto developed. In this way, horizontalalignment films in which a sufficient alignment regulating power wasdeveloped by the irradiation with light were obtained. The filmthickness after the post-baking was 100 nm.

Subsequently, the TFT substrate and the CF substrate were attachedtogether in the same manner as in Example 1, and the liquid crystalcomposition was sealed between the substrates. The alignment, filmsdescribed above contained carboxyl groups on the liquid crystal layerside because the polyamic acid was partially exposed on the surface. Thewidth of the seal material was 0.8 nm. A liquid crystal materialcontaining a compound including an alkoxy structure represented by aformula (E-3-1) below to which the same nitrobenzene derivative as inExample 1 represented by the formula (G-1-1) above was added was used asthe liquid crystal composition. The concentration of the nitrobenzenederivative was set to 200 ppm with respect to the total amount of theliquid crystal composition. The liquid crystal material had a negativeanisotropy of dielectric constant (Δε=−3.5).

Then, a pair of polarizing plates were attached to the back surface(surface on which light from a backlight is incident) side of the TFTsubstrate and the observation surface (surface from which light from abacklight is emitted) side of the CF substrate such that thepolarization axes were in a relationship of crossed Nicols, and thus aliquid crystal panel for the FFS mode was produced. Subsequently, abacklight including white LEDs was attached to the back surface side ofthe liquid crystal panel, and thus the liquid crystal display device ofExample 4 was completed.

COMPARATIVE EXAMPLE 6

A liquid crystal display device of Comparative Example 6 was produced inthe same manner as in Example 4 except that no nitrobenzene derivativewas added to the liquid crystal composition.

(Assessment Test 4)

A current was supplied, for 1000 hours, to the liquid crystal panelsproduced in Example 4 and Comparative Example 6 in the same manner as inAssessment Test 1, and the VHR before the supply of current (initialVHR) and after 1000 hours was measured. In addition, the screen of eachliquid crystal panel was visually observed, and presence of stain andimage sticking was checked. The results are shown in Table 4.

TABLE 4 VHR (%) Stain Image sticking VHR (%) (After (After (After(Initial) 1000 hours) 1000 hours) 1000 hours) Example 4 99.0 99.0 Notpresent Not present Comparative 99.0 98.0 Not present Slightly presentExample 6

As can be seen from Table 4, the decrease of VHR is larger inComparative Example 6 than in Example 4, and it is indicated that thedecrease of VHR was successfully suppressed by adding a nitrobenzenederivative to the liquid crystal material. In addition, in ComparativeExample 6, although no stain was observed, slight image sticking wasobserved. In contrast, in Example 4 in which a nitrobenzene derivativewas added to the liquid crystal material, no stain or image sticking wasobserved.

The cyclobutane structure included in the photo-alignment films used inExample 4 generates radicals in an intermediate stage of a reaction by,normally, mainly absorbing light having a wavelength of 300 nm orsmaller as shown in a reaction formula below. However, thephoto-alignment films including the cyclobutane structure may besometimes modified to a structure with a better light absorbingcharacteristic to reduce the amount of exposure to light at the time ofalignment treatment. In the case where such modification is performed,although the light absorbance for light on the longer wavelength sideincreases, there is a possibility that the short wavelength side of thelight emission spectrum of the back light overlaps the long wavelengthside of the absorption spectrum of the photo-alignment films. Inaddition, since the amount of exposure to light at the time of alignmenttreatment is as large as several hundreds of millijoules or larger persquare centimeter, there is also a possibility that part of radicalsgenerated in the alignment treatment is still not deactivated aftercompletion of the liquid crystal panel. Therefore, a cause of occurrenceof image sticking is also present in a photo-alignment film of adecomposition type including a cyclobutane structure, and it wasconfirmed that image sticking occurred in Comparative Example 6. Inaddition, from the results of Example 4, it was revealed that thenitrobenzene derivative effectively deactivates the radicals generatedby the reaction of the photo-alignment films and thus image sticking canbe prevented.

In addition, the alkoxy structure in the liquid crystal material used inExample 4 is preferably used for adjusting the degree of anisotropy ofdielectric constant of a liquid crystal material (negative liquidcrystal) having a negative anisotropy of dielectric constant. The degreeof anisotropy of dielectric constant of a liquid crystal material(positive liquid crystal) having a positive anisotropy of dielectricconstant can be easily adjusted without using the alkoxy group. Aconventional liquid crystal display device has a tendency that the VHRthereof decreases in the case of using a liquid crystal materialincluding an alkoxy structure, and this tendency is particularlyprominent in the case of using a photo-alignment film in combination.However, in the case of adding a nitrobenzene derivative as in thepresent invention, the decrease of VHR can be suppressed. The reason forthis can be explained using the following Hypothesis Models 1 to 5.

[Hypothesis Model 1]

An alkoxy structure (particularly, a methoxy group and a ethoxy group)is an electron-releasing group, and has a resonance structure whenexposed to light. The following formula represents part of a compoundhaving an alkoxy structure, and shows three resonance structurescorresponding to the alkoxy structure. Among these, a resonancestructure (a) shown at the center and a resonance structure (b) shown onthe right are ionic states, and thus cause the decrease of VHR. Further,the resonance structures (a) and (b) respectively change, in thepresence of oxygen, to structures (a′) and (b′) including a peroxidestructure. The structures (a′) and (b′) including a peroxide structureare easily turned into radicals as shown in (a″) and (b″). The decreaseof VHR is caused by ionization of the generated radicals.

[Hypothesis Model 2]

As shown in a reaction formula below, an alkoxy structure (—OR) islikely to be attacked by a radical R_(AL), generated in aphoto-alignment film, and causes radical generation reactions of fourpatterns. The decrease of VHR is caused by ionization of the generatedradicals.

[Hypothesis Model 3]

As shown in a reaction, formula below, the radical R_(AL) generated inthe photo-alignment film bonds to oxygen in the liquid crystal layer andforms a peroxide structure (ROO.). An alkoxy structure (˜OR) is likelyto be attacked by a peroxide structure, and causes radical generationreactions of five patterns. In addition, in each pattern, after aradical generation reaction, other radical generation reactions arerepeated like a chain reaction. The decrease of VHR is caused byionization of the generated radicals. It is to be noted that the radicalchain reaction going through a peroxide structure is known as anautomatic oxidation reaction.

[Hypothesis Model 4]

A negative liquid crystal including an alkoxy structure is constitutedby a molecular structure with a large polarization, and thus thesolubility of impurity ions therein is higher than in a positive liquidcrystal, and mobile ions are likely to be present in the liquid crystal.The mobile ions have an effect of canceling a charged electrical charge,and thus the VHR decreases as a result.

Radicals are involved in Hypothesis Models 1 to 3 described above, andthese models can be addressed by scavenging the radicals by a radicalscavenger. In addition, Hypothesis Model 4 explains that a positiveliquid crystal is influenced more than a negative liquid crystal byionic impurities generated through generation of radicals, andscavenging the radicals also indirectly addresses Hypothesis Model 4. Asdescribed above, an effect of suppressing the decrease of VHR caused inthe case of using a liquid crystal material including an alkoxystructure can be achieved by containing a radical scavenger in theliquid crystal layer.

[Hypothesis Model 5]

A compound including an alkoxy structure generates a radical by itselfby absorbing light (amount of energy: hv) such as ultraviolet light asrepresented by a formula below even without being subjected to transfer(attacking) of a radical generated in a photo-alignment film, forexample. The decrease of VHR is caused by ionization of the generatedradicals. A nitrobenzene derivative can absorb ultraviolet light.Further a nitrobenzene derivative can be concentrated more in thevicinity of an alignment film in the liquid crystal layer. Therefore, inthe case of adding a nitrobenzene derivative to the liquid crystalmaterial, the nitrobenzene derivative can absorb ultraviolet lightincident on the liquid crystal layer before a compound including analkoxy structure does. Therefore, even in the case where the amount ofaddition is small (in the order of several hundreds of parts permillion), generation of radicals from the compound including an alkoxygroup can be effectively suppressed, and stain and image sticking can beprevented.

EXAMPLE 5

A liquid crystal display device including a liquid crystal panel of theFFS mode was actually produced in the same manner as in Example 3. Asolid component of the alignment film solution was a polymer materialcontaining a polysiloxane structure as a main skeleton and cinnamategroups represented by the formula (B-1) above serving as photofunctionalgroups as side chains, and a polyamic acid.

The alignment film solution was applied to the surface of each of theTFT substrate and the CF substrate, and then both the substrates wereheated at 70° C. to volatilize the solvent in the alignment filmsolution. Subsequently, both the substrates were heated at 230° C. forpost-baking. Then, as photo-alignment treatment, the surfaces of boththe substrates were irradiated with linearly polarized light having amain wavelength of 313 nm at an intensity of 200 mJ/cm². Thepolarization direction of the linearly polarized light was set so as tobe perpendicular to the direction in which the liquid crystal was to bealigned. As a result of the irradiation with the linearly polarizedlight, the cinnamate groups underwent an isomerization reaction and adimerization reaction, and thus an alignment regulating power developed.In this way, horizontal alignment films in which a sufficient alignmentregulating power was developed by the irradiation with light wereobtained. The film thickness after the post-baking was 100 nm.

Subsequently, the TFT substrate and the CF substrate were attachedtogether in the same manner as in Example 1, and the liquid crystalcomposition was sealed between the substrates. The alignment filmsdescribed above contained carboxyl groups on the liquid crystal layerside because the polyamic acid was partially exposed on the surface. Thewidth of the seal material was 0.8 mm. A liquid crystal materialcontaining a compound having an alkenyl structure represented by theformula (D-1-1) above to which a nitrobenzene derivative represented bythe formula (G-1-1) above and an antioxidant represented by a formula(I-f) below were added was used as the liquid crystal composition. Theconcentration of the nitrobenzene derivative was set to 200 ppm withrespect to the total amount of the liquid crystal composition. Theconcentration of the antioxidant was set to 0.1 weight's with respect tothe total amount of the liquid crystal composition. The liquid crystalmaterial had a positive anisotropy of dielectric constant (Δε=+9.0).

Then, a pair of polarizing plates were attached to the back surface(surface on which light from a backlight is incident) side of the TFTsubstrate and the observation surface (surface from which light from abacklight is emitted) aide of the CF substrate such that thepolarization axes were in a relationship of crossed Nicola, and thus aliquid crystal panel for the FFS mode was produced. Subsequently, abacklight including white LEDs was attached to the back surface side ofthe liquid crystal panel, and thus the liquid crystal display device ofExample 5 was completed.

COMPARATIVE EXAMPLE 7

A liquid crystal display device of Comparative Example 7 was produced inthe same manner as in Example 5 except that no nitrobenzene derivativeor antioxidant was added to the liquid crystal composition.

(Assessment Test 5)

A current was supplied, for 1000 hours, to the liquid crystal panelsproduced in Example 5 and Comparative Example 7 in the same manner as inAssessment Test 1, and the VHR before the supply of current (initialVHR) and after 1000 hours was measured. In addition, the screen of eachliquid crystal panel was visually observed, and presence of stain andimage sticking was checked. The results are shown in Table 5.

TABLE 5 VHR (%) Stain Image sticking VHR (%) (After (After (After(Initial) 1000 hours) 1000 hours) 1000 hours) Example 5 99.0 98.0 Notpresent Not present Comparative 99.0 96.0 Present Present Example 7

As can be seen from Table 5, the result of comparison between Example 5and Comparative Example 7 indicates that the decrease of VHR wassuccessfully suppressed by adding a nitrobenzene derivative and anantioxidant to the liquid crystal material. In Comparative Example 7,stain and image sticking occurred. The stain and image sticking areconsidered to be caused by the decrease of VHR. In contrast, in Example5, such a malfunction was not observed as a result of adding anitrobenzene derivative and an antioxidant.

EXAMPLE 6

A liquid crystal display device including a liquid crystal panel of theFFS mode was actually produced in the same manner as in Example 3 exceptthat the width of the narrowest part of the seal material was 0.6 mm orsmaller when the TFT substrate and the CF substrate were attachedtogether.

COMPARATIVE EXAMPLE 8

A liquid crystal display device of Comparative Example 6 was produced inthe same manner as in Example 6 except that no nitrobenzene derivativewas added to the liquid crystal composition.

(Assessment Test 6)

A current was supplied, for 1000 hours, to the liquid crystal panelsproduced in Example 6 and Comparative Example 8 in the same manner as inAssessment Test 1, and the VHR before the supply of current (initialVHR) and after 1000 hours was measured. In addition, the screen of eachliquid crystal panel was visually observed, and presence of stain andimage sticking as checked. The results are shown in Table 6.

TABLE 6 VHR (%) Stain Image sticking VHR (%) (After (After (After(Initial) 1000 hours) 1000 hours) 1000 hours) Example 6 99.0 97.8 Notpresent Not present Comparative 99.0 94.7 Present Present Example 8(peripheral stain)

As can be seen from Table 6, the result of comparison between Example 6and Comparative Example 8 indicates that the decrease of VHR wassuccessfully suppressed by adding a nitrobenzene derivative to theliquid crystal material. In Comparative Example 8, as a result ofsetting the seal width to be 0.6 mm or smaller, moisture entered theliquid crystal panel from the outside, and the VHR decreasedprominently. In contrast, in Example 6, by adding a nitrobenzenederivative to the liquid crystal material, not only ionization ofradicals generated in the photo-alignment films was suppressed but alsoionization of neutral impurities by the moisture that entered the liquidcrystal panel from the outside was successfully suppressed, and, as aresult, it was revealed that the decrease of VHR could be suppressed.Whereas stain was observed in the periphery of the screen due to theentry of the moisture in Comparative Example 8, no stain (stain in theperiphery of the screen) or image sticking was observed in Example 6 inwhich a nitrobenzene derivative was added to the liquid crystalmaterial.

EXAMPLE 7

A liquid crystal display device including a liquid crystal panel of theFFS mode was actually produced in the same manner as in Example 3. Asolid component of the alignment film solution was a polymer materialcontaining a polysiloxane structure as a main skeleton and cinnamategroups represented by the formula (B-1) above serving as photofunctionalgroups as side chains, and a polyamic acid.

The alignment film solution was applied to the surface of each of theTFT substrate and the CF substrate, and then both the substrates wereheated at 70° C. to volatilize the solvent in the alignment filmsolution. Subsequently, both the substrates were heated at 230° C. forpost-baking. Then, as photo-alignment treatment, the surfaces of boththe substrates were irradiated with linearly polarized light having amain wavelength of 313 nm at an intensity of 20 mJ/cm². The polarizationdirection of the linearly polarized light was set so as to beperpendicular to the direction in which the liquid crystal was to bealigned. As a result of the irradiation with the linearly polarizedlight, the cinnamate groups underwent an isomerization reaction and adimerization reaction, and thus an alignment regulating power developed.In this way, horizontal alignment films in which an alignment regulatingpower was developed by the irradiation with light were obtained. Thefilm thickness after the post-baking was 100 nm. In this example, theamount of exposure to light at the time of photo-alignment treatment wasreduced compared with Example 5 in which the same alignment filmsolution was used, and, as will be described later, the alignmentregulating power was improved by causing a photopolymerizable monomeradded to the liquid crystal material to polymerize on the surfaces ofthe alignment films.

Subsequently, the TFT substrate and the CF substrate were attachedtogether in the same manner as in Example 1, and the liquid crystalcomposition was sealed between the substrates. The alignment filmsdescribed above contained carboxyl groups on the liquid crystal layerside because the polyamic acid was partially exposed on the surface. Thewidth of the seal material was 0.8 mm. A liquid crystal materialcontaining the same compound as in Example 1 having an alkenyl structurerepresented by the formula (D-1-1) above to which a photopolymerizablemonomer represented by a formula (C-3-1) below, a nitrobenzenederivative represented by the formula (G-1-1) above, and an antioxidantrepresented by the formula (I-f) above were added was used as the liquidcrystal composition. The amount of mixed photopolymerizable monomer wasset to 0.25 wt % with respect to the total amount of the liquid crystalcomposition. The concentration of the nitrobenzene derivative was set to200 ppm with respect to the total amount of the liquid crystalcomposition. The concentration of the antioxidant was set to 0.1 weight% with respect to the total amount of the liquid crystal composition.The liquid crystal material had a negative anisotropy of dielectricconstant (Δε=−3.5).

Monomers other than the monomer represented by the formula (C-3-1) abovemay be used as the photopolymerizable monomer.

For example, a monomer represented by the formula (C-1-1) above in whichthe skeleton part of the monomer represented by the formula (C-3-1)above is changed to biphenyl, and a monomer represented by the formula(C-1-2) above in which methacrylate groups at the terminal ends of themonomer represented by the formula (C-1-1) are changed to acrylategroups may be used. Further, in the formulae (C-1-1), (C-1-2), and(C-1-3), hydrogen atoms present at the skeleton part may be eachindividually substituted by a halogen atom.

After curing the seal material, a display region of the liquid crystalpanel was irradiated with black light at an intensity of 3000 mJ/cm². Asa result of this, the photopolymerizable monomer in the liquid crystallayer polymerized on the surfaces of the alignment films while involvingliquid crystal molecules. As a result of this, the alignment of theliquid crystal on the surfaces of the alignment films was fixed by thepolymer of the photopolymerizable monomer and thus a sufficientalignment regulating power was achieved.

Then, a pair of polarizing plates were attached to the back surface(surface on which light from a backlight is incident) side of the TFTsubstrate and the observation surface (surface from which light from abacklight is emitted) side of the CF substrate such that thepolarization axes were in a relationship of crossed Nicols, and thus aliquid crystal panel for the FFS mode was produced. Subsequently, abacklight including white LEDs was attached to the back surface side ofthe liquid crystal panel, and thus the liquid crystal display device ofExample 7 was completed.

COMPARATIVE EXAMPLE 9

A liquid crystal display device of Comparative Example 9 was produced inthe same manner as in Example 7 except that no nitrobenzene derivativeor antioxidant was added to the liquid crystal composition.

(Assessment Test 7)

A current was supplied, for 1000 hours, to the liquid crystal panelsproduced in Example 7 and Comparative Example 9 in the same manner as inAssessment Test 1, and the VHR before the supply of current (initialVHR) and after 1000 hours was measured. In addition, the screen of eachliquid crystal panel was visually observed, and presence of stain andimage sticking was checked. The results are shown in Table 7.

TABLE 7 VHR (%) Stain Image sticking VHR (%) (After (After (After(Initial) 1000 hours) 1000 hours) 1000 hours) Example 7 99.0 98.0 Notpresent Not present Comparative 99.0 96.0 Present Present Example 9

As can be seen from Table 7, the result of comparison between Example 7and Comparative Example 9 Indicates that the decrease of VHR wassuccessfully suppressed by adding a nitrobenzene derivative and anantioxidant to the liquid crystal material. In addition, stain and imagesticking were observed in Comparative Example 9. The stain and imagesticking are considered to be caused by the decrease of VHR. Incontrast, in Example 7, such a malfunction was not observed as a resultof adding a nitrobenzene derivative.

The photopolymerizable monomer used in Example 7 and Comparative Example9 serves as a radical generation source. Accordingly, in Example 7 andComparative Example 9, the photopolymerizable monomer is present as aradical generation source in addition to the photo-alignment films, andradicals are likely to be generated in the liquid crystal layer. Withregard to this, by adding a nitrobenzene derivative to the liquidcrystal material, not only radicals generated by a reaction of thephoto-alignment films but also the photopolymerizable monomer remainingafter PSA treatment can be effectively deactivated. In addition, asimilar effect can be achieved by adding an antioxidant to the liquidcrystal material. For the reasons above, although stain and imagesticking occurred in Comparative Example 9, the stain and image stickingcould be effectively prevented in Example 7.

EXAMPLE 8

A liquid crystal display device of Example 8 was produced in the samemanner as in Example 3 except that a compound including one nitrobenzeneskeleton in one molecule represented by a formula (H-1-1) below was usedas the nitrobenzene derivative.

(Assessment Test 8)

A current was supplied, for 1000 hours, to the liquid crystal panelproduced in Example 8 in the same manner as in Assessment Test 1, andthe VHR before the supply of current (initial VHR) and after 1000 hourswas measured. In addition, the screen of each liquid crystal panel wasvisually observed, and presence of stain and image sticking was checked.The results are shown in Table 8.

TABLE 8 VHR (%) Stain VHR (%) (After (After Image sticking (Initial)1000 hours) 1000 hours) (After 1000 hours) Example 8 99.0 98.2 Notpresent Not present

As can be seen from Table 8, in Example 8 in which a compound includingone nitrobenzene skeleton in one molecule was used as the nitrobenzenederivative, the decrease of VHR could be suppressed. In addition, stainand image sticking could be effectively prevented.

[Additional Remarks]

An embodiment of the present invention may be a liquid crystal displaydevice of the present invention that includes an active matrix liquidcrystal panel and a backlight, the liquid crystal panel including aliquid crystal layer, a pair of substrates that sandwich the liquidcrystal layer in between, and an alignment film disposed on a liquidcrystal layer side surface of each of the substrates, each alignmentfilm being a photo-alignment film formed from a material that exhibits aphoto-alignment characteristic and containing carboxyl groups on theliquid crystal layer side, the liquid crystal layer containing a liquidcrystal material and a nitrobenzene derivative represented by a formula(1) below. According to the embodiment described above, radicals elutedinto the liquid crystal layer can be efficiently scavenged anddeactivated by a nitrobenzene derivative, and thus the decrease of VHRcan be prevented. As a result of this, a good VHR can be retained for along period of time by using a photo-alignment film, and occurrence ofimage sticking and stain in a display screen can be prevented.

(In the formula, X represents a monovalent organic group.)

It is preferable that the nitrobenzene derivative is a compoundrepresented by a formula (2-1), (2-2), (2-3), (2-4), or (2-5) below. Thecompound represented by the formula (2-1), (2-2), (2-3), (2-4), or (2-5)below has a high reactivity with radicals, and thus can quicklydeactivate radicals in the liquid crystal layer.

(In the formulae, m represents an even number from 2 to 18.)

It is preferable that the nitrobenzene derivative is a compoundrepresented by a formula (3-1), (3-2), (3-3), or (3-4) below.

(In the formulae, n represents an integer from 1 to 18.)

The liquid crystal layer may further contain an antioxidant representedby a formula (4) below. By containing the antioxidant represented by theformula (4) below, groups such as alkyl groups (R) contained in theliquid crystal, alignment films, and seal material can be prevented frombeing oxidized by oxygen entering the liquid crystal panel, and radicalsgenerated from an oxidized matter thereof can be prevented from causingthe decrease of VHR.

(In the formula, X represents a monovalent organic group.)

Examples of the photo-alignment films include photo-alignment filmsincluding at least one photoreactive part selected from the groupconsisting of cinnamate, chalcone, coumarin, stillbene, azobenzene, andphenol ester. In addition, the photo-alignment films may be polymersobtained by polymerizing a monomer containing an acid anhydriderepresented by a formula (5) below. The long wavelength side of anabsorption spectrum of these photo-alignment films overlaps the shortwavelength side of a light emission spectrum of the backlight, and thesephoto-alignment films generate radicals by being irradiated with lightfrom the backlight. Accordingly, the effect of preventing the decreaseof VHR when a nitrobenzene derivative is applied can be achievedsufficiently.

(In the formula, any of hydrogen atoms may be substituted).

A compound including an alkenyl structure may be used as at least onecomponent of the liquid crystal material, and examples of the compoundincluding an alkenyl structure include compounds represented by aformula (6-1), (6-2), or (6-3) below. Although a liquid crystalcomponent including an alkenyl structure is effective for reducing theviscosity of a liquid crystal material, a double bond included in thealkenyl structure is likely to be attacked by a radical. Accordingly,the effect of preventing the decrease of VHR when a nitrobenzenederivative is applied can be achieved sufficiently.

(In the formulae, m and n represent the same or different integers.)

The liquid crystal material may have a negative anisotropy of dielectricconstant. Conventionally, there is a tendency that the malfunctions ofimage sticking and stain appear more prominently when a liquid crystalmaterial having a negative anisotropy of dielectric constant is usedthan when a liquid crystal material having a positive anisotropy ofdielectric constant is used. Accordingly, the effect of preventing thedecrease of VHR when a nitrobenzene derivative is applied can beachieved more sufficiently.

At least one component of the liquid crystal material may be a compoundincluding an alkoxy structure, and examples of the compound including analkoxy structure include compounds represented by a formula (7-1),(7-2), (7-3), (7-4), or (7-5) below. An alkoxy structure (particularly,a methoxy group and an ethoxy group) includes an ionic state in aresonance structure thereof, and thus causes the decrease of VHR.Accordingly, it is desired that further decrease of the VHR is preventedby applying a nitrobenzene derivative.

(In the formula, m and n represent the same or different integers.)

As the alignment mode of the liquid crystal panel, a 4-domain reverse TNmode, a fringe field switching mode, or an in-plane switching mode ispreferably used.

In alignment treatment for the 4D-RTN mode, the FFS mode, and the IPSmode, it is required to control the azimuth of the liquid crystalalignment with a high precision, and thus photo-alignment treatment ispreferably used. A nitrobenzene derivative can remarkably prevent thedecrease of VHR when used in combination with a photo-alignment film. Inaddition, the amount of radiation in photo-alignment treatment of ahorizontal alignment mode is normally larger than in the case of avertical alignment mode by one digit or more, and more radicals arelikely to be generated by a side reaction than in the case of thevertical alignment mode. Therefore, the effect of preventing thedecrease of VHR can be sufficiently achieved when a nitrobenzenederivative is applied.

The liquid crystal panel may include, on a liquid crystal layer sidesurface of each of the alignment films, a layer containing a polymerobtained by a photopolymerizable monomer represented by a formula (8)below, and examples of Y in the formula (8) mentioned above includestructures represented by a formula (9-1), (9-2), or (9-3) below. In thecase where a photopolymerizable monomer is added to the liquid crystallayer for PSA treatment, the photopolymerizable monomer serves as aradical generation source in addition to the photo-alignment films, andthus radicals are likely to be generated in the liquid crystal layer.Accordingly, the effect of preventing the decrease of VHR when anitrobenzene derivative is applied can be achieved sufficiently.A1-Y-A2  (8)(In the formula, Y represents a structure including at least one benzenering and/or condensed benzene ring; any of hydrogen atoms in the benzenering and condensed benzene ring may be substituted by a halogen atom; atleast one of A1 and A2 represents acrylate or methacrylate; and A1 andA2 are directly bonded to the benzene ring or the condensed benzenering.)

(In the formulae, any of hydrogen atoms may be substituted by halogenatoms).

The embodiments of the present invention shown above may be combined asappropriate within the spirit of the present invention.

REFERENCE SIGNS LIST

-   10: backlight-   20: liquid crystal panel-   21: substrate-   22: alignment film-   23: liquid crystal layer

The invention claimed is:
 1. A liquid crystal display device comprising:an active matrix liquid crystal panel; and a backlight, the liquidcrystal panel including a liquid crystal layer, a pair of substratesthat sandwich the liquid crystal layer in between, and an alignment filmdisposed on a liquid crystal layer side surface of each of thesubstrates, each alignment film being a photo-alignment film formed froma material that exhibits a photo-alignment characteristic and containingcarboxyl groups on the liquid crystal layer side, the liquid crystallayer containing a liquid crystal material and a nitrobenzene derivativerepresented by a formula (2-1), (2-2), (2-3), (2-4), or (2-5) below:

wherein m represents an even number from 2 to
 18. 2. The liquid crystaldisplay device according to claim 1, wherein the liquid crystal layerfurther contains an antioxidant represented by a formula (I-1), (I-2),(I-3), (I-a), (I-b), (I-c), (I-d), (I-e), or (I-f) below:

(In the formula, n represents an integer)


3. The liquid crystal display device according to claim 1, wherein thephoto-alignment films each contain at least one photoreactive partselected from the group consisting of cinnamate and azobenzene.
 4. Theliquid crystal display device according to claim 1, wherein thephoto-alignment films are polymers obtained by polymerizing a monomercontaining an acid anhydride represented by a formula (5) below:

wherein any of hydrogen atoms may be substituted.
 5. The liquid crystaldisplay device according to claim 1, wherein at least one component ofthe liquid crystal material is a compound including an alkenylstructure.
 6. The liquid crystal display device according to claim 5,wherein the compound including an alkenyl structure is a compoundrepresented by a formula (6-1), (6-2), or (6-3) below:

wherein m and n represent the same or different integers.
 7. The liquidcrystal display device according to claim 1, wherein the liquid crystalmaterial has a negative anisotropy of dielectric constant.
 8. The liquidcrystal display device according to claim 7, wherein at least onecomponent of the liquid crystal material is a compound including analkoxy structure.
 9. The liquid crystal display device according toclaim 8, wherein the compound including an alkoxy structure is acompound represented by a formula (7-1), (7-2), (7-3), (7-4), or (7-5)below:

wherein m and n represent the same or different integers.
 10. The liquidcrystal display device according to claim 1, wherein an alignment modeof the liquid crystal panel is a 4-domain reverse TN mode, a fringefield switching mode, or an in-plane switching mode.
 11. The liquidcrystal display device according to claim 1, wherein the liquid crystalpanel includes, on the liquid crystal layer side surface of eachphoto-alignment film, a layer containing a polymer obtained bypolymerizing a photopolymerizable monomer represented by a formula (8)below:A1-Y-A2  (8) wherein Y represents a structure containing at least onebenzene ring and/or condensed benzene ring; any of hydrogen atoms in thebenzene ring and condensed benzene ring may be substituted by a halogenatom; at least one of A1 and A2 represents acrylate or methacrylate; andA1 and A2 are directly bonded to the benzene ring or the condensedbenzene ring.
 12. The liquid crystal display device according to claim11, wherein Y in the formula (8) represents a structure represented by aformula (9-1), (9-2), or (9-3) below:

wherein any of hydrogen atoms may be substituted by halogen atoms.